Problem after upgrade from 1.5.1 to 1.5.2.1 with Admin

I tried an upgrade from 1.5.1 to 1.5.2.1 and it all seemed to go fine until it came to logging into my admin and every time I typed the URL in it redirected to the install so, maybe stupidly, I uploaded the install folder again to see if I could sort the problem out but now I just keep getting this error message;

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /home/autobet/public_html/install/model/upgrade.php on line 63
Could not Execute: DROP TABLE return_product;
Unknown table 'return_product'

My site (http://autobetting.co.uk) does actually appear to be fine but I simply can't get admin access any more and have absolutely no idea how to rectify this problem.

OK so an update to this is that I created the table in PHPMyadmin (or however it is written) and it dropped the table and said that the upgrade was completed successfully but then when I click on the log into your admin option is still redirects to the install folder which obviously isn't there anymore.

Can't work this out for nothing TBH.

The other problem that I now face is that my site is in maintenance mods and I can't access admin to do anything about it.

Hasn't anyone had this happen before or got any idea why this would happen?

Getting desperate now.